Thiol compounds play a principal role in maintaining the appropriate oxidation-reduction state of proteins, cells and organisms. Thiols present in biological systems include glutathione, cysteine, N-acetylcysteine (AcCSH), and protein thiols, as well as the contribution of these components to symmetric and mixed disulfides.
Glutathione is the principal intracellular non-protein thiol and plays a major role in the maintenance of the intracellular red-ox state. Glutathione exists in a dimerized oxidized state (GSSG) and a reduced monomeric state (GSH).
Glutathione, as with other thiol compounds, is a nucleophilic scavenger and an electron donor via the sulfhydryl group. Its reducing ability maintains molecules such as ascorbate and proteins in their reduced state. Glutathione is also the cofactor for the selenium-containing glutathione peroxidases. These enzymes detoxify peroxides, such as hydrogen peroxide and other peroxides. Another antioxidant activity of glutathione is the maintenance of the antioxidant/reducing agent ascorbate in its reduced state. This is accomplished via glutathione-dependent dehydroascorbate reductase which is comprised of glutaredoxin and protein isomerase reductase. Glutathione also plays roles in catalysis, metabolism, signal transduction, gene expression and apoptosis.
Glutathione (GSH) indicators exist, including monochlorobimane (mBCl), and 4-chloromethylcoumarin (CMAC), and are traditionally used for detection of intracellular GSH as an indicator of apoptosis.
Compounds having a general quinolizinocoumarin scaffold and functionalized versions thereof have been used as fluorescent labels, such as for carbonyl or malonaldehyde detection (Traore et al. J. of Chromatography, 648(1) (1993) pgs. 111-118), glucose biosensors (WO 05/65241), detection agents for human aldo-keto 1C reductases (AKR1Cs) (WO 06/23821), and for derivatizing amines (Tod et al. Analytica Chimica Acta, 223(2) (1989) pgs 309-17).
In order to accurately detect glutathione and other thiol species in a biological sample it would be desirable to have a highly fluorescent thiol indicator compatible with biological systems.